B. 10 should be the correct answer.
Reason : As one is primary key and it varies from 1 to 10 , So using one atmost 10 tuples can be there. When we take two into consideration it varies from 1 to 5 and is unique, which implies it can be NULL. Hence the number of tuples will be 10, with two being NULL in five of them.
CORRECT ME IF I AM WRONG.